The present invention relates to network systems having redundant routers for receiving packets from a host on a LAN. More specifically, the invention provides a "standby group" of routers including an "active" router which handles packets from the host and a "standby" router which backs up the active router should it fail.
Local area networks (LANs) are commonly connected with one another through one or more routers so that a host (a PC or other arbitrary LAN entity) on one LAN can communicate with other hosts on different LANs. Typically, the host recognizes only those addresses for the entities on its LAN. When it receives a request to send a data packet to an address that it does not recognize, it communicates through a router which determines how to direct the packet between the host and the address. Unfortunately, a router may, for a variety of reasons, become inoperative (e.g., a power failure, rebooting, scheduled maintenance, etc.). When this happens, the host communicating through the inoperative router may still remain connected to other LANs if it can send packets to another router connected to its LAN.
Various protocols have been devised to allow a host to choose among routers in a network. Two of these, Routing Information Protocol (or RIP) and ICMP Router Discovery Protocol are examples of protocols that involve dynamic participation by the host. The host in a RIP system receives the periodic routing protocol packets broadcast by the various routers on the system and thereby keeps track of available routers. If a router stops sending protocol packets, the host assumes that the router is no longer operative and stops sending data through that router. Unfortunately, routing protocol packets contain relatively large amounts of data including all the specific routes known by the routers. Because the host periodically receives these rather large packets, the system bandwidth is reduced.
In ICMP Router Discovery, the host keeps track of operative routers by listening for router reachability messages. These messages contain a list of IP addresses of usable routers together with preference values for those routers. Because these messages are relatively small (in comparison to routing protocol packets received by the host in RIP) and are not coupled time-wise with any routing protocol, the bandwidth utilization is improved in comparison with RIP. Nevertheless, both RIP and Router Discovery require that the host be dynamically involved in the router selection, thus reducing performance and requiring special host modifications and management.
In a widely used and somewhat simpler approach, the host recognizes only a single "default" router. In this approach, the host is configured to send data packets to the default router when it needs to send packets to addresses outside its own LAN. It does not keep track of available routers or make decisions to switch to different routers. This requires very little effort on the host's part, but has a serious danger. If the default router fails, the host can not send packets outside of its LAN. This will be true even though there may be a redundant router able to take over because the host does not know about the backup. Unfortunately, such systems are now widely used in mission critical applications such as stock trading.
Other systems in which the host becomes overly dependent upon a single router have similar problems. For example, in a "proxy ARP" protocol, a router may give a host its address in response to the host's request for an address outside of its local LAN. Thereafter, the host directs its traffic through that router. If the host does not often update its ARP table entry (which lists physical addresses of available routers), it may continue to assume that it should send all data packets through the same router, even after that router fails. Unfortunately when this happens, the host can no longer communicate outside its own LAN.
In view of the above, it would be desirable to have a network system in which the hosts are not dynamically involved in router selection, and yet are able to handle failures by an assigned router.